1. Field of the Invention
The present invention relates to a method for casting such dental metals as titanium designed to make plate frames, clasps, etc. in the dental technical field, which enables high-quality products to be cast with neither casting deficiencies nor casting cavities by melting dental metal ingots by arc discharge.
2. Statement of the Prior Art
In the dental technical field, titanium has been used for CAST PLATES, clasps, etc., because it is light in weight, has a certain or more strength and excels in corrosion resistance and bioaffinity.
So far, metal frames, etc. used for, e.g. crowns or porcelain-fused-to-metal crowns have been cast by melting dental metals in the atmosphere with the use of suitable techniques such as high-frequency melting and pouring the molten metals in molds, because the dental metals are noble metal alloys relatively difficult to oxidize. Since the above-mentioned titanium has the property of being likely to undergo oxidization, however, it should be melted in an atmosphere of such an inert gas as argon. To meet such demand, reliance has been placed on a casting technique wherein dental metal ingots are melted by arc discharge and the obtained molten metals are poured in molds.
According to this casting technique, an arc electrode is disposed above in a hermetically sealed melting chamber, and a crucible made of an electrically conductive material is positioned just below the arc electrode. With the arc electrode and crucible connected to a cathode and an anode, respectively, the dental metal ingot to be cast is first placed on the crucible. After evacuation to vacuum, the melting chamber is then filled with such an inert gas as argon until its internal pressure amounts to a pressure nearly equal to atmospheric pressure. Subsequently, the ingot is melted by arcs occurring from the arc electrode. Finally, the molten metal is poured in a mold chamber through the inlet of a mold located in it, said mold chamber being partitioned from said melting chamber by a partition wall provided with a through-hole formed in its portion positioned below said crucible.
When titanium is used with this casting technique, it should be rapidly poured into the mold through its inlet, partly because its melting point is higher than those of common dental metals, and partly because it has to be cast with the mold maintained at room temperature, so that it cools down and solidify rapidly--due to the fact that it reacts with mold material at such a high temperature as in casting common noble metal alloys. In other words, titanium must be poured under pressure into the mold by increasing pressure in the melting chamber to make a pressure difference between the melting chamber and the mold chamber.
The mold used is constructed from investment material composed of a binder/aggregate combination. More specifically, the binder and aggregate are kneaded together with water or an exclusive liquid into slurry. After that, a wax model is invested in the slurry, followed by burning out wax at about 700.degree. C. in furnace.
Owing to having some air permeability, such investment material may be used for casting noble metal alloys forming part of metal frames, etc. used for crowns or porcelain-fused-to-metal crowns. This is because if it is pressurized with a pressure difference made between the melting and mold chambers, then an increase in the in-mold gas pressure is so suppressed by its air permeability that the molten dental metal can be well poured into the mold.
Among common investment materials, however, there is a gypsum-bonded investment material using gypsum as the binder. At 700.degree. C. or higher, this material increases in air permeability and decreases in heat resistance, since gypsum decomposes thermally at that temperature and so cannot retain its crystal form. For instance, when a high temperature dental metal like titanium is cast with that investment material, they react with each other to evolve such amount of gases that the gases cannot escape only through the inherent air permeability the investment material inherently has. This would increase the pressure in the casting region of the mold, making it impossible to pour the predetermined amount of the molten dental metal in the mold. Otherwise gases would be entrained in the molten metal, giving rise to cavities. In order to cast a high temperature dental metal, use has thus been made of a phosphate-bonded investment material using a phosphate as the binder. With this phosphate type of a material in which the binder phosphate combines with a metal oxide to form an amorphous product, gas emissions are limited even upon heated at high temperatures, since it undergoes no or little change in state and is of increased heat resistance. However, this leads to another defect that its air permeability is badly limited. For that reason, it has been proposed to use coarse aggregates for increased air permeability, thereby reducing a pressure rise in the casting regions of molds when the molten dental metal are poured in the casting regions. However, this method makes the surfaces of casting products rough, nonetheless, fails to obtain sufficient air permeability.
Thus, when no sufficient air permeability is obtained, the internal pressure in the casting region of the mold is increased too much to pour the predetermined amount of the molten dental metal into the mold. This gives rise to such disadvantages as casting deficiencies and entrainment of gases in the molten metal, resulting in casting cavities.
Arc melting of a dental metal ingot at around atmospheric pressure causes arcs to concentrate on its local points under the influences of magnetic blow, etc., often making its uniform melting impossible and thereby causing it to be locally heated to high temperatures. When the dental metal ingot melts in its entirety, the molten dental metal reacts with the crucible material. In order to prevent this, additional special mechanisms for moving the electrodes are needed.
In order to obviate the above-mentioned defects of the prior art, the present invention seeks to provide a method for casting dental metals, which can give well-smoothened castings by stable arc melting with no fear of causing casting deficiencies or cavities.